As a known important process of synthetic fiber production, a synthetic fiber tow which was imparted with false crimps by a crimping machine is subjected to heat treatment in a heat treating machine being provided with a pressure chamber filled with steam or other heating medium.
Several means were proposed to charge the synthetic fiber tow (referred also to as "crimped fiber lump" hereafter) into the pressure chamber in the as-crimped state or in an extended form and to draw out the crimped fiber lump from the pressure chamber while maintaining the pressure inside of the pressure chamber.
One of the means to maintain the pressure inside of the pressure chamber is to fill the pressure seal parts at the entrance and exit of the pressure chamber with the crimped fiber lump. Generally, this type of heat treating machine feeds the crimped fiber lump by sandwiching it between a pair of endless conveyor belts from upper and lower sides thereof (the conveyor is referred to simply as "sandwiching conveyor belts" hereafter). For this type of pressure seal method, the seal is obtained only by giving a higher friction resistance between the crimped fiber lump and the sandwiching endless conveyor belts at the pressure seal part to stop the leak of pressure in the pressure chamber.
To increase the friction resistance aforedescribed and to prevent the leak of steam or other heating medium from the pressure chamber, the apparent density of crimped fiber lump at the entrance of the pressure chamber is required to increase. However, the introduction of a crimped fiber lump having a high apparent density into the pressure chamber results in the increased fiber packing density on the sandwiching endless conveyor belts, which then requires a long period of heat treatment and results in an extended heat treating machine. In addition, within the pressure chamber, the crimped fiber lump develops thermal shrinkage along with the progress of heat treatment to decrease the cross sectional area thereof at the exit pressure seal part of the pressure chamber.
Consequently, the reduction of vertical resistance reduces the friction resistance between the crimped fiber lump and the sandwiching endless conveyor belts. Then, the pressure seal at the seal part is lost to blow out the crimped fiber lump to the outside of the exit pressure seal part of the pressure chamber. As a result, the pressure chamber becomes difficult to maintain its inside pressure.
To improve the disadvantage, a crimped fiber lump is extended before charging it into the pressure chamber. This method completes the heat treatment in a shorter time than the case of non-extended crimp fiber lump because it has less fiber packing density on the sandwiching endless conveyor belts than that of non-extended crimp fiber lump. Nevertheless, the method is unable to solve the problems that the crimped fiber lump is blown out from the exit pressure seal part and that the treating fiber suffers damage.
As a result, presently the method to fill both the entrance and exit pressure seal parts of the pressure chamber with the crimped fiber lump is not often adopted. Instead, a labyrinth seal is usually applied to the exit pressure seal parts. The labyrinth seal method has, however, disadvantages such that the method induces frequent fiber damages caused by the direct friction action of high temperature treating fiber on the labyrinth part, that the method accepts only the fiber which extended its crimps in advance, and that an increased pressure needs a deep labyrinth seal part. Thus, the labyrinth method is not a satisfactory one.
An alternative and improved method to maintain pressure inside of the pressure chamber is what is called "roller seal method". The method arranges a set of nip rollers at the entrance and/or exit pressure seal part to hold down the crimped fiber lump to give a sufficient pressure seal. In this method, however, in particular at the exit pressure seal part, the crimped fiber lump at an elevated temperature passes through a high hold down pressure zone, which induces fusing of filaments and damage on single filaments to result in a quality defect. A modified method to the roller seal method employs a cooling chamber at the inlet of the exit roller seal part to cool the crimped fiber lump before entering the seal part. This modification also has disadvantages such that the facilities become complex and need an elongated space and that the quality of treated fiber is not satisfactory.
As described above, there is no satisfactory method which prevents the extension of facilities, has an excellent pressure stability inside of the pressure chamber, and solves the quality problems such as damage of heat treated fiber.
The object of this invention is to provide a method for maintaining pressure inside of the pressure chamber while solving the problems described above relating to a continuous heat treating machine and maintaining a stable pressure inside of the pressure chamber without deteriorating the treated fiber, and responding to varied treatment conditions.